These vast spinning discs of hydrogen, tens of thousands of light-years across, weigh more than a billion of our Suns. And each has a tiny number of barely visible stars at the centre.

The total mass of these galaxies is less than our Milky Way galaxy, around which our Sun and all of its nine planets orbit. "Compared to other galaxies, they are low mass," said Brad Warren, a graduate student in astronomy at the Australian National University in Canberra who led the study. His team discovered 20 of the 'gassy' galaxies in total.

The starlight in the gassy galaxies is a small proportion of the total galaxy, he explained: "When you look for gas [in these galaxies], the signal just booms in. But when you look for stars, all you see is a barely recognisable smudge."

This is not the first time galaxies with a high proportion of gas have been seen. But the new ones are dramatically 'gassy', with a ratio of about five times as much gas as the stars. "These have 10 to 15 times the amount of gas as they do stars," Warren told ABC Science Online.

Gas in space is detectable at particular radio frequencies, so Warren's team searched the sky for the most gaseous galaxies using the Parkes Radiotelescope and the Australia Telescope five-dish array in rural New South Wales. They then followed each of them up with optical observations using the ANU 2.3-metre Telescope at the Siding Spring Observatory near Coonabarabran.

"A lot of surveys in the past started with optical data that was then followed up by radio data," Warren said. "We started with a blind survey of the southern sky, which allowed us to go the other way. We took the results from the radio data, to see if there was a galaxy there. You are more likely to find them that way around."

It is not known why these 'empty galaxies' have not transformed their rich source of hydrogen gas into masses of stars, as most galaxies do. Hydrogen is the most common element in the universe, forming the building block from which stars and all other matter form.

"Most galaxies, like our own Milky Way, have transformed most of their gas into stars, but the galaxies we have discovered have held back - and we are not sure why," said Warren. "Discovering this missing link will give us important insights into how, when and why galaxies, such as our own, formed."

One suggestion is that there is something in each galaxy that is preventing stars from forming, such as a large amount of nearby dark matter. Another is that because these galaxies are isolated, there has been no catalyst for star formation. "If two galaxies pass each other, the gravitational pull on each other can cause star formation," said Warren.

Results of the research, done in collaboration with university colleague Dr Helmut Jerjen and Dr Baerbel Koribalski of Australia's Commonwealth Scientific & Industrial Research Organisation, are considered preliminary, said Warren. The next step is to take the optical, radio and infrared observations and investigate the new galaxies fully.